The 68000 takes 4 clock cycles (minimum) to access memory and can read or write 2 bytes. At 8 MHz that means that the 68000 has a memory bandwidth of (8Mclocks/

The 68000 takes 4 clock cycles (minimum) to access memory and can read or write 2 bytes. At 8 MHz that means that the 68000 has a memory bandwidth of (8Mclocks/s/4clocks)x2bytes = 4Mbytes/s. The 6502 takes 1 clock cycle (which might be stretched if needed) to access memory and can read or write 1 byte; At 2 MHz the 6502 in the BBC has a memory bandwidth of 2Mbytes/s. So going from the 6502 to the 68000 would only get you twice the performance. But the 68000 takes many more clock cycles to handle an interrupt compared to the 6502 and since the BBC Micro was designed around interrupts instead of DMA this would eat up all the performance gain. This was true of the 16 bit micros from National, Zilog and Intel as well. The ARM2 takes 1 clock cycle (which can be stretched if needed) to access memory and can read or write 4 bytes. In addition, it can tell memory if the next access will be random or if it will be at the address following the current one. DRAM has to slow down the processor for random accesses (a RAS cycle followed by a CAS cycle) but can work at full speed for sequential accesses (just CAS cycles). That means that sometimes the ARM2 runs at 4 MHz and sometimes at 8 MHz - supposing an average of 6 MHz is very reasonable. So we are talking about a bandwidth of 6Mx4 = 24Mbytes/s. That is 12 times faster than the 6502 and 6 times faster than the 68000. And the ARM2 has a normal interrupt that is very fast plus a fast interrupt that is instantaneous. Note that in theory you need "2MHz" DRAMs for the BBC's 6502 or an 8MHz 68000. But in practice if you want to share the memory between video and the CPU you will need 4MHz DRAMs. Which is what the ARM2 also uses, but without sharing with video. The solution is to fetch the video in chunks using the sequential CAS-only mode, put it in a FIFO and then feed the video that instead of directly. Which is what the MEMC/VIDC companion chips for the ARM2 did. Now the video still slows down the processor, but doesn't hog 50% of the bandwidth.